As one of spark plugs used for ignition of internal combustion engines, such as an automobile gasoline engine, there is a type of spark plug in which an insulator having a stem portion of which diameter is reduced toward the front end side through a stepped portion is mounted on a main metal member by an engagement of the stepped portion with an engaging projection portion formed on an inner circumferential surface of the main metal member, and in which a clearance is formed between the inner circumferential surface of the engaging projection portion and an outer circumferential surface of the stem portion, which is opposed thereto. In such a spark plug, once the clearance is invaded by an unburned gas generated, for example, under low-temperature environment, etc., the outer circumferential surface of the stem portion in the clearance is stained. As a result, stain resistance of the spark plug is spoiled.
Thus, there is proposed a spark plug that can be designed to have a small size without spoiling stain resistance of the spark plug, by narrowing the clearance to have a predetermined distance or less, as opposed to a prior art common knowledge to make the clearance wider in view of stain resistance.
For example, in Patent Publication 1 (Japanese Patent Application Publication No. 2002-260817), there is described a spark plug characterized by that it is equipped with a cylindrical main metal member (1), an insulator (2) that is locked on an inner circumferential side of the main metal member (1) and has an axial hole, a center electrode (3) retained in the axial hole of the insulator (2), and an earth electrode that forms a spark discharge gap (g) by facing to an end of the center electrode (3), that, in an axial direction (O) of the insulator (2), a side on which the spark discharge gap (g) is positioned is defined as a front side, and a side opposite to this is defined as a rear side, that the insulator (2) has a front end portion (2i) of which diameter is reduced by a circumferential stepped portion that is defined as an insulator side engaging portion (2h), that the insulator is inserted into the main metal member (1) from a rear side opening portion, that the insulator side engaging portion (2h) is engaged with a metal member side engaging portion (1c) that projects from an inner circumferential surface of the main metal member (1), that an outer circumferential surface (hereinafter referred to as a clearance-forming outer circumferential surface) (2k) of the portion (2i), which is positioned on the front side of the insulator side engaging portion (2h) of the insulator (2), is opposed to an inner circumferential surface (hereinafter referred to as a clearance-forming inner circumferential surface) (52) of the metal member side engaging portion (1c) in a manner to form an engaging position clearance (Q) of a predetermined amount, and that, when an outer diameter of the clearance-forming outer circumferential surface (2k) is defined as d1 and when an inner diameter of the clearance-forming inner circumferential surface (52) is defined as D1, an amount β of the engaging position clearance, which is expressed by β=(D1−d1)/2, is adjusted to 0.4 mm or less.
Furthermore, in Patent Publication 2 (Japanese Patent Application Publication No. 2005-183177), there is described a spark plug characterized by that it is equipped with an insulator that is generally cylindrically formed and has a through hole in an axial direction, a rod-shaped center electrode that is inserted into a front end side of the through hole of the insulator, a generally cylindrical main metal member that retains it by inserting a front end side in an axial direction of the insulator, and an earth electrode that is joined at one end portion to a front end of the main metal member, faces to the center electrode at the other end portion opposite to the one end portion, and forms a spark discharge gap between the other end portion and the center electrode, that the insulator is made up of an insulator rear end portion formed on a rear end side of the insulator, an insulator front end portion that is formed on a front end side of the insulator and has a diameter reduced from an outer diameter of the insulator rear end portion, and a first insulator stepped portion for connecting together the insular rear end portion and the insulator front end portion, that the main metal member is made up of a main metal member rear end portion formed on a rear end side of the main metal member, a main metal member front end portion that is formed on a front end side of the main metal member and has, on at least a rear end side, a portion having an inner diameter reduced from an inner diameter of the main metal member rear end portion, and a first main metal member stepped portion for connecting together the main metal member rear end portion and the main metal member front end portion, that the first insulator stepped portion is engaged with the first main metal member stepped portion through a packing, that, when a section containing an axis is viewed, an outer diameter of the insulator front end portion is represented by d1 and an inner diameter of the main metal member front end portion is represented by D1, and that a length, which is parallel to the axial direction of the insulator, of a clearance satisfying “(D1−d1)/2<0.45 mm” is 1.2 mm or more and 5 mm or less, when the front end side of the insulator is defined as plus, as a position on the front end side in the axial direction of engagement positions between the packing and the main metal member stepped portion is defined as a starting point,
By the way, in order to make a small-size spark plug, it is effective to make the insulator thin in thickness, besides making the clearance narrow. On the other hand, when the clearance is made narrow, there is a tendency in general that the voltage applied to the insulator, which forms the clearance, becomes high. Therefore, it becomes possible to have a small-size spark plug, while maintaining its stain resistance, by using an insulator thinned in thickness and by making the clearance narrow. Due to a high voltage applied to this insulator, however, insulation breakdown may occur. As a result, withstand voltage characteristics may lower. In particular, when a condition with a high voltage applied to the insulator continues for a long term, withstand voltage characteristics may lower significantly due to the occurrence of many insulation breakdowns on the insulator. For example, when a spark plug is made small such that nominal diameter of the mounting thread portion becomes M12 or less, the spark plug becomes difficult to have both stain resistance and withstand voltage characteristics.